The present invention relates to a microwave filter to be used in microwave integrated circuits, in particular a band rejection or low-pass filter.
In transmission paths in microwave integrated circuits there is of course a need for filtering elements. In particular there may be a need for filters the characteristics of which can be varied, such as a filter having a filtering effect only for a specific state of a control signal. Very compact microwave filters can be built using high-temperature cuprate superconductors using e.g. planar stripline structures. Such filters are used in high-performance radio communication systems, e.g. as microwave receiving filters for radio base stations, in which filter having very sharp skirts and low insertion losses as well as small sizes and small weights which are important.
In the Japanese patent application JP 2/101801, a microwave band-rejection filter is disclosed having transmission lines designed as linear microstrip, metal elements placed on top of an area of a layer of superconducting material. The superconducting material area has a pattern substantially agreeing with that of the metal conductor, except in some regions where the width of the superconducting area is larger than that of the metal conductor. When the superconducting material is in a non-superconducting state, most of the electric current passes through the common metal material of the metal conductor, whereas, in superconducting state, the electrical current passes only through the superconducting underlying material. The microstrip metal elements thereby obtain a variable filtering effect. However, a disadvantage of this design resides in providing a region having some, though it may be low, electrical conductivity placed under the normal conductor, since this region causes losses in the transmission line. The conductivity of materials, which are superconducting at a low temperature and are suitable for microwave integrated circuits, have in their normal state an electrical conductivity corresponding to some 10xe2x88x923 to 10xe2x88x922 times that of the electrical conductivity of the material of the always normal metal conductor.
It is an object of the invention to provide a switchable filter based on a microstrip transmission line for microwaves, the filter exhibiting low losses.
Thus, a low-pass or band-rejection filter for e.g. microwave frequencies is designed as a substantially planar structure and is constructed of transmission lines designed as linear microstrip elements which have widths which are varied by making areas at the sides of the linear elements superconducting. In changing the widths of the transmission lines also the inductances thereof are changed accordingly. The areas at the sides of the microstrip elements comprise rather narrow areas located directly at the central, normal metal conductor and are thus electrically connected thereto along at least portions of the sides or of the edges of the central microstrip elements. These narrow areas have in the non-superconducting state some electrical conductivity which can be small but still not quite insignificant in relation to that of the metal conductor. However, due to the fact that they contact the central, always normal metal conductor only at very low or thin edges thereof instead of contacting it at a large surface they do no significantly affect the transmission characteristics of the transmission path in the normal state of those areas which can be made superconducting. The transmission lines also comprise capacitance areas which contribute to their capacitance. The capacitance areas project laterally from central stem elements of the transmission lines and are portions of the central, normal metal conductor and are thus made from a normal electrically conducting material which can not be made superconducting at the considered temperatures.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.